This invention relates to ignition timing control devices for controlling the control timings, such as ignition timings, of the ignition system of an internal combustion engine, and more particularly to such ignition control devices which determines the igntion timings, etc., of a multicylinder engine on the basis of a single rotational position signal representing the position of the crankshaft of the engine in relation to the respective cylinders.
It is becoming increasingly common to utilize microcomputers for the control of the ignition system of an automotive engine. In such case, the rotational position of the crankshaft of the engine is detected by a rotational position signal genrator which, for example, generates pulses whose leading and trailing edges correspond, respectively, to the first and second positions of the crankshaft in relation to the resepctive cylinders. FIG. 1 shows an example of the waveform of a rotaional position signal P generated by such a position signal generator, in the case where the engine is a four-cylinder four-stroke engine. Thus, pulses corresponding to the position of the crankshaft with respect to cylinders No. 1, No. 3, No. 4, and No. 2, for example, are generated successively in that order, during two revolutions (rotation of 720 dgrees) of the crankshaft during which complete cycles of suction, compression, expansion, and exhaution of respective cylinders are completed. The leading edge of each pulse corresponds to the first rotational position of the crankshaft, which, for example, is at 75 degrees before the top dead center (BTDC) between the compression and the expansion stroke with respect to each cylinder; likewise, the trailing edge of each pulse corresponds to the second rotational position of the crankshaft, which is, for example, at 5 degrees before the top dead center between the compression and the expansion stroke of each cylinder. The pules corresponding to the respective cylinders are generated at the pulse repetition period of 180 degrees.
The ignition timings, etc., of respective cylinders are controlled on the basis of the position signal P of FIG. 1; let us describe this control operation.
First, referring to FIG. 2, let us explain the case where the first positions of the crankshaft to which the leading edges of the pulses correspond are utilized as the reference points. In such case, the supply of current C to the ignition coil is controlled as follows. Let t1 be the time point at which the leading edge of a pulse is detected; then, the ignition timing T1 is given by: EQU T1=t1+Tal`,
where Ta1 is the interval of time which the crandshaft takes to rotate from the first position (e.g., 75 BTDC) to the ignition angle (e.g., 8 degrees ATDC (after top dead center)). On the other hand, the supply of energization current C to the ignition coil is started at the energization starting time T2 given by: EQU T2=t1+Tdw1
where Tdw1 is the interval of time which should lapse from the first rotational position of the crankshaft to the starting of the energization of the ignition coil for the purpose of the current ignition.
The case where the control is effected on the basis of the second positions of the crankshaft is shown in FIG. 3. Let t2 be the time point at which the trailing edge of a pulse is detected; further let Ta2 be the interval of time which the crankshaft takes to rotate from the second rotational position (i.e., 5 degrees BTDC) to the ignition angle (e.g., 8 degrees ATDC); then, the ignition timing T3 is given by : EQU T3=t2+Ta2.
Further, let Tdw2 be the interval of time which should lapse from the second rotational position of the crankshaft to the commencement of energization of the ignition coil for the purpose of the next ignition; then, the energization starting timing T4 for the next ignition is given by: EQU T4=t2+Tdw2.
In the case where a rotational position signal P as shown in FIG. 1 is utilized, a separate pulse signal (i.e., a cylinder identifying signal) for determining the correspondance of the pulses with respective cylinders is necessary. Thus, recently, a proposition had been made to utilize a rotational position signal P having a waveform as shown in FIG. 4. In the case of position signal P of FIG. 4, the second position of the crankshaft with respect to a specific cylidner (e.g. cylinder No. 1) is displaced by an offset angle .theta. (e.g., 10 degrees) to the retarding direction, so that the trailing edges of the pulses corresponding to the specific cylinder is, for example, 5 degrees after the top dead center (ATDC), instead of at 5degrees before the top dead center (BTDC); hence, the pulses corresponding to the specific cylinder can be identified on the basis of the determination of the duty ratio t/T (i.e., the ratio of the pulse width t to the pulse repetition period T).
In the case where the position signal P as shown in FIG. 4 is utilized, the following problem may occur: Let us assume that the second positions of the crankshaft are utilized as reference points in the determination of the control timings such as the ignition timings and the energization stating timings of the ignition coil. Then, as shown in FIG. 5, the ignition timing and the energization starting timing that are determined on the basis of the trailing egde of the pulse corresponding to the specific cylinder are displaced to the retarding direction by an amount corresponding to the offset angle .theta.. Thus, the ignition timing of the specifc cylinder is retarded by an angle 74 (e.g., 10 degrees) with respect to the normal ignition timing, which retardation reduces the performance of the engine. In addition, the duration of the energization of the ignition coil as started in response to the trailing edge of the pulse corresponding to the specific cylinder is decreased by a length of time corresponding to the offset angle .theta., so as to reduce the ignition voltage; this may result in a misfiring.